This invention relates to an amphoteric ion exchanger made of a straight-chain multi-block copolymer which is composed of a macromolecular chain A.sup.- having a cation exchange group, a macromolecular chain B.sup.+ having an anion exchange group and another macromolecular chain C having no ion exchange group with these constituent chains bonded in such a manner that the chains A.sup.- and B.sup.+ are separated from each other by the chain C.
Materials that simultaneously have cation and anion exchange groups have been developed as mosaic amphoteric ion exchange membrane to be used mainly for making sea water into fresh water by piezo-dialysis or salt concentration thereof. Known methods for obtaining such an amphoteric ion exchange membrane include the use of a blend of two or more kinds of homopolymers, a binary or ternary block copolymer, a blend of two kinds of random copolymers, a blend of a block copolymer and a homopolymer, etc.
Normally, mixing a macromolecule having a positive fixed electric charge (poly-cation) and a macromolecule having a negative fixed electric charge (poly-anion) gives a complex of macromolecular electrolyte (a poly-ion complex).
In such a case, the function expected of an amphoteric ion exchanger disappears and the poly-ion complex thus obtained is not usable as membrane material for the above stated piezo-dialysis. Therefore, generally, in obtaining an amphoteric ion exchanger either from a polymer blend or from a block copolymer, first a homopolymer or a block copolymer having no ion exchange group is shaped as desired and then cation and anion exchange groups are introduced. In this case, as well known, macromolecules of different kinds or constituent macromolecules of different kinds forming a block copolymer do not mix with each other and stay in their own domains. It is possible to obtain a desired amphoteric ion exchanger by introducing cation and anion exchange groups into the respective domains of such a material. Known amphoteric ion exchangers that have been obtained in this manner include:
(1) A blend of poly-styrene and poly(2-vinyl pyridine). PA1 (2) A block copolymer of styrene and 2-vinyl styrene. PA1 (3) A ternary block copolymer consisting of styrene, 2-vinyl pyridine and methyl methacrylate which are combined in that order. PA1 (4) A blend consisting of a random copolymer of styrene and isoprene and a random copolymer of 2-vinyl pyridine and isoprene. PA1 (5) A ternary block copolymer consisting of styrene, p-vinyl benzyl dimethyl amine and isoprene which are combined in that order. PA1 (6) A ternary block copolymer consisting of isoprene, 2-vinyl pyridine and t-butyl acrylate which are combined in that order.
NOTE
(1)-(4): OSW R & DP Report No. 689 PA0 (5)-(6): A Japanese Patent Application, Laid Open No. 56-76408
Among the monomers mentioned above, the styrene permits introduction of a cation exchange group by sulfonation thereof; and the methyl methacrylate and the t-butyl acrylate permit introduction of a cation exchange group by hydrolysis. Meanwhile, the 2-vinyl pyridine and the p-vinyl benzyl dimethyl amine permit introduction of an anion exchange group by quaternization. Further, it has been attempted to enhance the strength of the membrane by crosslinking the isoprene portion thereof.
With the polymer blends and block copolymers employed in accordance with the conventional known method, however, the strength of the material is insufficient. Therefore, swelling by a solvent or the like tends to cause pinholes or cracks either during the introduction of the ion exchange group or after the introduction thereof. In some worse cases, the whole membrane comes to collapse. According to the experiences of the present inventors, even if such a trouble can be avoided with an ordinary binary or ternary block coplymer, there still might take place a change in the micro-phase separated structure to make it difficult to obtain a sufficiently stable performance as amphoteric ion exchanger for some applications.
In each of the above stated block copolymers, a block consisting of monomer units which introduce a cation exchange group and a block consisting of monomer units which introduce an anion exchange group are adjacently interbonding. Naturally, in their micro-phase separated solid state, a domain introducing a cation and a domain introducing an anion are considered adjoining each other.
Generally, a thin compatible region is formed at an interface between two domains consisting of different micro-phase separated monomer units. Therefore, introduction of a cation and an anion gives a polyion complex formed at the interfacial region. It is readily conceivable that a function as amphoteric ion exchanger greatly degrades in such a case.